The vertebrate immune response is comprised of adaptive and innate immunity components. While the adaptive immune response is selective for particular pathogens and is slow to respond, components of the innate immune response recognize a broad range of pathogens and respond rapidly upon infection. One such component of the innate immune response is the complement system.
The complement system includes about 20 circulating proteins, synthesized primarily by the liver. Components of this particular immune response were first termed “complement” due to the observation that they complemented the antibody response in the destruction of bacteria. These proteins remain in an inactive form prior to activation in response to infection. Activation occurs by way of a pathway of proteolytic cleavage initiated by pathogen recognition and leading to pathogen destruction. Three such pathways are known in the complement system and are referred to as the classical pathway, the lectin pathway and the alternative pathway. The classical pathway is activated when an IgG or IgM molecule binds to the surface of a pathogen. The lectin pathway is initiated by the mannan-binding lectin protein recognizing the sugar residues of a bacterial cell wall. The alternative pathway remains active at low levels in the absence of any specific stimuli. While all three pathways differ with regard to initiating events, all three pathways converge with the cleavage of complement component C3. C3 is cleaved into two products termed C3a and C3b. Of these, C3b becomes covalently linked to the pathogen surface while C3a acts as a diffusible signal to promote inflammation and recruit circulating immune cells. Surface-associated C3b forms a complex with other components to initiate a cascade of reactions among the later components of the complement system. Due to the requirement for surface attachment, complement activity remains localized and minimizes destruction to non-target cells.
Pathogen-associated C3b facilitates pathogen destruction in two ways. In one pathway, C3b is recognized directly by phagocytic cells and leads to engulfment of the pathogen. In the second pathway, pathogen-associated C3b initiates the formation of the membrane attack complex (MAC). In the first step, C3b complexes with other complement components to form the C5-convertase complex. Depending on the initial complement activation pathway, the components of this complex may differ. C5-convertase formed as the result of the classical complement pathway comprises C4b and C2a in addition to C3b. When formed by the alternative pathway, C5-convertase comprises two subunits of C3b as well as one Bb component.
Complement component C5 is cleaved by either C5-convertase complex into C5a and C5b. C5a, much like C3a, diffuses into the circulation and promotes inflammation, acting as a chemoattractant for inflammatory cells. C5b remains attached to the cell surface where it triggers the formation of the MAC through interactions with C6, C7, C8 and C9. The MAC is a hydrophilic pore that spans the membrane and promotes the free flow of fluid into and out of the cell, thereby destroying it.
An important component of all immune activity is the ability of the immune system to distinguish between self and non-self cells. Pathology arises when the immune system is unable to make this distinction. In the case of the complement system, vertebrate cells express proteins that protect them from the effects of the complement cascade. This ensures that targets of the complement system are limited to pathogenic cells. Many complement-related disorders and diseases are associated with abnormal destruction of self cells by the complement cascade. In one example, subjects suffering from paroxysmal nocturnal hemoglobinuria (PNH) are unable to synthesize functional versions of the complement regulatory proteins CD55 and CD59 on hematopoietic stem cells. This results in complement-mediated hemolysis and a variety of downstream complications. Other complement-related disorders and diseases include, but are not limited to autoimmune diseases and disorders, neurological diseases and disorders, blood diseases and disorders and infectious diseases and disorders. Experimental evidence suggests that many complement-related disorders are alleviated through inhibition of complement activity. Therefore, there is a need for the development of compounds capable of selectively blocking complement-mediated cell destruction.